Advances in the development and improvements of the luminous flux of light-emitting devices such as solid-state semiconductor and organic light-emitting diodes (LEDs) have made these devices suitable for use in general illumination applications, including architectural, entertainment, and roadway lighting. Light-emitting diodes are becoming increasingly-competitive with light sources such as incandescent, fluorescent, and high-intensity discharge lamps.
One of the challenges in solid-state lighting is to design a system and/or method that can set and maintain intensity and chromaticity of the mixed light emitted by a plurality of color, for example, blue and yellow or red, green, and blue LEDs. This can be challenging as the light emitted by LEDs may vary depending on operating conditions other than the electrical currents provided to the LEDs. Traditionally, systems that can rectify this dependency employ optical feedback based on signals provided by one or more optical sensors. The sensors can sense a portion of the emitted light and can be used to determine the chromaticity and the intensity of the sensed light. In turn, information about the chromaticity and intensity can be used to adjust the drive currents of the LEDs accordingly. However, a number of effects must be addressed to enable effective feedback control. For example, firstly, the spectral responsivities of known cost-effective RGB color sensors do not, for practical purposes, sufficiently closely mimic the spectral responsivity of the human eye. Secondly, the spectral power distributions (SPDs) of the LEDs can change with LED operating temperature.
For example, FIG. 1 illustrates the normalized spectral responsivity of a standard human observer as represented by the CIE color matching functions x(λ), y(λ), z(λ) along with the responsivity of typical commercially available RGB color sensors. It is clearly visible that the sensor characteristics do not closely match those of the standard human observer. Spectral mismatches, even smaller than the ones illustrated, can cause undesired light effects in feedback-controlled multi-color LED based systems.
As is well known in the art an SPD described by Φ(λ) can be transformed into corresponding CIE tristimulus values by determining the averages of the SPD weighted with the corresponding color matching functions. This can be expressed in the following equations for the above noted CIE color matching functions:X=k∫Φ(λ) x(λ)dλ  (1a)Y=k∫Φ(λ) y(λ)dλ  (1b)andZ=k∫Φ(λ) z(λ)dλ  (1c)
As such tristimulus values determined based on signals provided by RGB color sensors with insufficiently accurate responsivities may not provide practically useful indications of the CIE tristimulus values. As is well known, other color matching functions may be used to determine the respective stimuli in the respective color space.
Known solutions such as exemplified by U.S. Pat. No. 6,507,159 disclose a method and a system for controlling a luminaire based on RGB LEDs that track the tristimulus values of both feedback and reference in a specific way. The forward currents driving the LED luminaire are adjusted based on a comparison between feedback tristimulus values and reference tristimulus values until the comparison yields no difference between the two. The tristimulus values are determined using certain filter sensor combinations. Matching the filters and sensors to accurately reproduce the CIE color matching functions, even under temperature-controlled laboratory conditions, however, is complex. Therefore, useful filter sensor combinations can be expensive, which are discussed by G. P. Eppeldauer, “A Reference Tristimulus Colorimeter,” Proceedings of the Ninth Congress of the International Color Association of the Optical Engineering Society, SPIE 4421, pp 749-752, (2002), Bellingham, Wash., USA. Furthermore, feedback control that is only based on CIE tristimulus values does not separate chromaticity (i.e. color) from intensity and therefore may not be effective in suppressing a number of undesired chromaticity fluctuations.
B. T. Barnes describes in “A Four-Filter Photoelectric Colorimeter,” Journal of the Optical Society of America 29, (10), pp 448-452, (1939), how to split the color matching function x(λ) into xl(λ) and xs(λ) by wavelength range and how this simplifies the spectral responsivity requirements for RGB sensors. Barnes defines: xS(λ)=0 and  xL(λ)= x(λ) if λ>504 nm  (2a) xS(λ)= x(λ) and  xL(λ)=0 if λ<504 nm  (2b)where l and s stand for long and short wavelength region. For other than laboratory-quality instruments, it is common practice in the prior art to use appropriately scaled versions of the blue filter-detector pair response to represent both the xs(λ) and z spectral responsivities. This approach, however, in general does not address how to mitigate undesired effects of RGB sensor spectral responsivity mismatches during operation.
B. A. Wandell and J. E. Farrell describe in “Water into Wine: Converting Scanner RGB to Tristimulus XYZ” Device-Independent Color Imaging and Imaging Systems Integration, Proc. SPIE 1909, pp 92-101, (1993), how to transform RGB sensor data into XYZ tristimulus values by using a transformation matrix that can be predetermined from a least squares solution during a calibration step. The calibration step utilizes data from ideal CIE color matching sensors and calibration data from non-ideal RGB sensors are obtained from measurements of a set of SPDs per sensor. However, Wandell do not teach the use of the least-squares solution with a real-time feedback apparatus, or its application to light source control. The transformation is only applied to the measured RGB color sensor data of each pixel of an image.
G. D. Finlayson and M. S. Drew describe in “Constrained Least-Squares Regression in Color Spaces,” Journal of Electronic Imaging 6, (4), pp 484-493, (1997), a method similar to the solution by Wandell et al. above that suffers from the same limitations.
FIG. 2 illustrates an example of the SPDs of light emitted by a RGB LED module at two different operating temperatures but otherwise the same static operating conditions. The ambient temperature is once 25° C. and once 70° C. Further to the effects of different operating temperature, different LED drive currents in different color LEDs can result in different rates of power dissipation and consequently different LED junction temperatures. This can manifest when comparing the SPDs in that different peak wavelengths shift and different SPDs broaden differently and hence can cause the chromaticity of the mixed light to change in a nonlinear fashion depending on the drive currents and the operating temperatures of each LED. In addition, thermal coupling between different color LEDs can cause interdependencies between the LED junction temperatures. Consequently, the well-known Grassman laws of color additivity may not provide accurate descriptions of the color of the mixed light without consideration of self and cross heating effects of the LEDs and any optical sensors employed to sense the generated light.
Luminaire feedback control systems can therefore suffer from a number of effects including the issue that RGB sensors with different sensitivities will provide different unique responses to light of the same SPD. Changes in the SPDs of color LEDs as described above will also cause variations in the responses of RGB sensors. Hence, variations of RGB sensor signals in response to variations of the SPD will also be unique. Furthermore, RGB sensors that approximate ideal sensors will, in response to the same SPD, provide different signals compared to ideal sensors. Furthermore, the responsivity of an RGB sensor may also vary with its temperature.
Therefore there is a need for a luminaire control system and method that can effectively control the light generated by a luminaire.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.